Piasecki's experience with tandem rotors continued with the H-21 Shawnee, evolved from the earlier HRP Rescuer with which it shared the nickname "Flying Banana" due to its fuselage shape. The H-21 was an all-metal design with counter-rotating three-blade tandem rotors. In its original USAF specification, it was designed for use in difficult terrain and climate conditions, particularly for Arctic operations. It was later adopted by the US Army which used it for a more traditional transport role where it was capable of carrying 20 fully loaded troops. Other additions included external fuel tanks and a belly sling. The H-21 was used extensively from the start of the Vietnam War where some were fitted door-mounted defensive machine guns (the first fitted on a helicopter) and others modified as gunships. It was the main helicopter used by the US until the arrival of the UH-1. Other operators included France (which employed it in a combat role in Algeria), Canada (license-built by Vertol Canada), West Germany (license-built by Weger Flugzeugbau), Japan, and Sweden.

First flight of the XH-21 prototype on 11 April 1952 and entered service with the USAF as the H-21A Work Horse (an earlier test variant was known as the YH-21. An upgraded engine was fitted to the H-21B with a rescue version known as the SH-21B. The first US Army version was the H-21C Shawnee and accounted for the bulk of production. All cargo versions were redesignated CH-21 and rescue versions HH-21 after 1962. A Canadian version built by Vertol Canada was known as the CH-127.